In this continuation proposal, we seek to expand and refine our understanding of the interaction between HIV-1 envelope glycoprotein and its coreceptor CCR5. During the last funding period, we confirmed our initial observation of the presence of tyrosine sulfation in the N-terminus of CCR5 and its critical role in HIV-1 entry. We also demonstrated that sulfated peptides based on the sequence of CCR5 N- terminus can block HIV-1 entry in human PBMC and macrophages and have begun to map in detail the regions in HIV-1 envelope glycoprotein that interact with CCR5 N-terminus. Finally, we have documented the presence and role of analogous sulfate moieties on CXCR4. In this renewal, will continue to investigate the molecular details of the association between HIV-1 envelope glycoprotein and the HIV-1 coreceptors. The following specific aims are offered to accomplish this goal. In Specific Aim 1, we will further analyze the molecular interactions of regions of gp120 with individual domains of CCR5, using the Ig-fusion constructs and biosulfated CCR5 peptides we have developed. In Specific Aim 2, we will examine the HIV-1 fusion process by utilizing _NR5 (N-terminal deleted CCR5) and sulfated CCR5 peptides. We have demonstrated during last funding period that this _NR5 does not support HIV-1 entry by itself, but does when complemented with N sulfated CCR5 peptides, offering an excellent system to dissect the complex HIV-1 fusion process. In Specific Aim 3, we seek to document the levels and regulation of CCR5's tyrosine sulfation in primary HIV-1 target cells. We will also study the contribution of tyrosine sulfation to the internalization and recycling of CCR5 in response to its chemokine ligands, a process criticalto chemokine mediated inhibition of HIV-1 entry.